Musnad Ahmad 31:127

Musa narrated to us, Ibn Lahi'ah narrated to us, from Ubayd Allah ibn al-Mughirah, from Abu al-Haytham, from Abu Sa'id al-Khudri, who said: "It is as if I am looking at the whiteness of the flank of the Messenger of Allah, may Allah bless him and grant him peace, while he was prostrating."